1. Field of the Invention
The present invention relates to a sheet feeding apparatus included in an image forming apparatus, which forms images on sheets, such as a copying machine and a laser beam printer and an image forming apparatus which has the sheet feeding apparatus.
2. Description of the Related Art
Conventionally, a sheet feeding apparatus provided in an image forming apparatus generally feeds the upper most sheet of sheets stacked on a lifting/lowering plate sequentially to an image forming portion by means of a feeding roller. This sheet feeding apparatus is configured to apply a force to a lifting/lowering plate provided to rock, in a feeding roller direction by means of a coil spring, and press the uppermost surface of the sheets stacked on the lifting/lowering plate against the feeding roller.
Further, there is a sheet feeding apparatus which has feeding cams fixed coaxially with a feeding roller, and which is configured such that the feeding cams push a lifting/lowering plate to a certain position, against the biasing force of a coil spring together with the lifting/lowering plate while the feeding roller rotates to feed out sheets (see U.S. Pat. No. 5,253,854).
Thus, the sheet feeding apparatus having a mechanism for pushing the lifting/lowering plate by means of the feeding cams can push the lifting/lowering plate to a certain position upon stand-by. This facilitates a setting and an exchange of sheets, and enhances the separation property of sheets in a separating portion such as a separating pad by pushing the lifting/lowering plate while sheets are sent out.
Further, there is an image forming apparatus on which the above sheet feeding apparatus is mounted (see Japanese Patent Laid-Open No. 2008-015077). An operation of this sheet feeding apparatus from feeding of sheets, forming of images to discharging of the sheets in an image forming apparatus will be schematically described with reference to FIG. 11.
As illustrated in FIG. 11, sheets S set on a sheet tray 1 are sent out when driving of a driving motor 16 is transmitted to rotate a feeding roller 2, and are separated by a separating pad.
The sheets S which are fed are conveyed to a transferring nip including a photosensitive drum 8 and a transfer roller 9 forming an image transferring portion through conveying rollers 3a and 3b. A lifting/lowering plate 22 provided to the sheet tray 1 can be lifted and lowered and is applied an upward force by a feeding spring 23. Cams 21 are provided coaxially with the feeding roller 2, and are in slidable contact with cam followers 22b provided in the lifting/lowering plate 22. Then, when the feeding roller 2 rotates, the cams 21 rotate to lift and lower the lifting/lowering plate 22 through the cam followers 22b, and the lifting/lowering plate 22 is lifted to press the sheets S against the feeding roller 2 to convey the sheets S.
By contrast with this, an image writing laser scanner 5 draws an electrostatic latent image on the photosensitive drum 8 in a process cartridge 7 to prepare for toner images. A toner image formed on the photosensitive drum 8 is transferred as a non-fixed image to the sheet S by the transferring nip including the photosensitive drum 8 and the transfer roller 9. In order to heat-fix this non-fixed image, the sheet S is sent to a fixing roller 11 to heat-fix the non-fixed image. The sheet S on which the image is fixed is sent to a discharge roller 12 along a conveying guide 15, and is discharged to a discharge tray 14.
However, with the above conventional example, when a rotating speed of the feeding roller 2 is increased to increase a sheet feeding speed, or when the diameter of the feeding roller 2 is made smaller to miniaturize a sheet feeding apparatus, the speed of rocking movement of the lifting/lowering plate 22 increases. As a result, there are a concern that noise becomes large when sheets abut on the feeding roller 2 and a concern that, when a feeding operation is repeated, an impact produced when sheets abut on the feeding roller 2 misaligns the stacked sheets. When sheets are misaligned, sheets are skewed while being fed and there is an issue of causing, for example, poor image quality and jamming.
The present invention provides a sheet feeding apparatus which suppresses a lifting speed of a lifting/lowering plate by providing an idling section between a feeding roller and feeding cams and stably feeds sheets with little noise upon a high-speed operation, and an image forming apparatus having this sheet feeding apparatus.